Yup. another thing that can affect cooling is the condenser... Many times they need to be cleaned... I will describe the process for coleman (and all others).
Remove the top cover 4 screws on a Coleman. (Around a dozen on most others).
Set aside.. on a coleman peal off the crud brush off what's left. and replace the cover.. Done. (on all others) remove the inner cover over the condenser (This may be a major job or just a few screws).. You may need to remove the fan (or not depending on too many things) then you peal and brush..
WHile you have the cover off clean everythign else that needs cleaning and as Dougrainer suggested.. Clamp on ammeter the compressor leads